Our goal is to elucidate the roles of amplitude and frequency of pulsatile luteinizing hormone (LH) secretion in regulation the steroidogenic function of the testis. The general hypotheses underlying the research are that both amplitude and frequency of secretion determine the effects of LH and that they do so specifically by means of the dynamic patterns of changes they produce in circulating LH concentrations. The long-term positive and negative effects of LH on testicular steroidogenesis, beyond its acute stimulation, are known from studies using heterologous hormone preparations given in supraphysiological doses via modes of administration that exposed the testis to nonphysiological patterns of the hormone. The proposed research is designed especially to investigate the roles of physiological LH secretory pulse amplitude and frequency in determining the positive, trophic effects of the hormone, but the same model could be used to study negative effects and their possible physiological significance. Experimental animals will be adult rams in which LH secretion has been suppressed chronically or acutely by active or passive immunization against LH-releasing hormone (LHRH). Rams will be infused intravenously with ovine LH in well-defined regimens of amplitude and frequency, and effects on several endpoints related to the steroidogenic function of the testis will be evaluated: responsiveness to acute stimulation, concentration of LH receptors, activities of certin key enzymes in the testosterone biosynthetic pathway, and number and structure of Leydig cells (the cells responsible for testosterone production). Responsiveness will be evaluated, in terms of both steroidogenic capacity and sensitivity, as acute responses to LH in vivo and to LH and a cyclic AMP analog in vitro. In vivo responses will be thoroughly characterized as to lag-time, magnitude, and duration. LH receptors will be quantified by means of a radioreceptor assay based on binding of the LH analog human chorionic gonadotropin. Enzyme activities will be measured by radiochemical precursor-product assays. Number and structure of Leydig cells will be evaluated by light and electron microscopic morphometry. Knowledge gained from these studies should have application in the control of male fertility and in the treatment of infertility resulting from inappropriate secretion of LHRH by the hypothalamus and/or of LH by the pituitary gland.